Observed drought trends in the United States, with hatching indicating a significant trend. Source: USGCRP (2009)

Water resources are important to both society and ecosystems. We depend on a reliable, clean supply of drinking water to sustain our health. We also need water for agriculture, energy production, navigation, recreation, and manufacturing.

Many of these uses put pressure on water resources, stresses that are likely to be exacerbated by climate change. In many areas, climate change is likely to increase water demand while shrinking water supplies. This shifting balance would challenge water managers to simultaneously meet the needs of growing communities, sensitive ecosystems, farmers, ranchers, energy producers, and manufacturers.

In some areas, water shortages will be less of a problem than increases in runoff, flooding, or sea level rise. These effects can reduce the quality of water and can damage the infrastructure that we use to transport and deliver water.

Impacts on Water Cycle and Water Demand

The water cycle (shown in the following figure) is a delicate balance of precipitation, evaporation, and all of the steps in between. Warmer temperatures increase the rate of evaporation of water into the atmosphere, in effect increasing the atmosphere's capacity to "hold" water.[1]Increased evaporation may dry out some areas and fall as excess precipitation on other areas.

Changes in the amount of rain falling during storms provide evidence that the water cycle is already changing. Over the past 50 years, the amount of rain falling during the most intense 1% of storms increased by almost 20%.[1]Warming winter temperatures cause more precipitation to fall as rain rather than snow. Furthermore, rising temperatures cause snow to begin melting earlier in the year. This alters the timing of streamflow in rivers that have their sources in mountainous areas.[1]

As temperatures rise, people and animals need more water to maintain their health and thrive. Many important economic activities, like producing energy at power plants, raising livestock, and growing food crops, also require water. The amount of water available for these activities may be reduced as Earth warms, and if competition for water resources increases.[1]

The Colorado River system is a major source of water supply for the Southwest. It supplies water for more than 30 million people in the cities of Los Angeles, Phoenix, Las Vegas, and Denver. Recent droughts, reductions in winter precipitation, and warmer, drier springs have caused water supplies in Colorado River reservoirs to decrease. Expected climate change impacts on Colorado River water supply include:

Increased year-to-year changes in water storage in reservoirs are possible, even under current conditions.

Decreased hydropower. For every 1% decrease in streamflow in the Colorado River Basin, there is a 3% decrease in hydroelectric power generation for the region.

Reductions in river discharge and runoff from snowmelt. Annual snowmelt runoff could also shift to earlier in the spring.

Impacts on Water Supply

Matching photographs taken 18 months apart during the most serious period of recent drought show a significant decrease in Lake Powell. Source: USGCRP (2009)

Many areas of the United States, especially the West, currently face water supply issues. The amount of water available in these areas is already limited, and demand will continue to rise as population grows. The West has experienced less rain over the past 50 years, as well as increases in the severity and length of droughts; this has been especially of concern in the Southwest.[1]

In the western part of the United States, future projections for less total annual rainfall, less snowpack in the mountains, and earlier snowmelt mean that less water will likely be available during the summer months when demand is highest. This will make it more difficult for water managers to satisfy water demands throughout the course of the year.[2][3]

The quality of water supply in coastal and island regions is at risk from rising sea level and changes in precipitation. Rising sea level and the occurrence of drought can increase the salinity of both surface water and ground water through salt water intrusion.

For example, the freshwater Everglades currently recharge Florida's Biscayne aquifer, a natural underground area that collects water and is the primary water supply to the Florida Keys. If rising sea levels submerge low-lying areas of the Everglades, portions of the aquifer would become saline. Sea level rise can also push salty water upstream in coastal areas, threatening surface water supplies. Aquifers in New Jersey east of Philadelphia are recharged by fresh portions of the Delaware River, which become saline during severe droughts.

Freshwater resources on some islands, especially small islands and atolls, can be limited, as supply depends on shallow aquifers, which are recharged by precipitation. These freshwater lenses float on top of the saltwater, and rising sea level diminishes the area above sea level in which the lens can reside. (For more detailed information,see the illustration on page 158 of this Climate Change Science Program Report (PDF)). Sea level rise can turn these shallow aquifers brackish through saltwater intrusion and droughts reduce the water available from other sources, further stressing these limited water supplies.

The Biscayne Aquifer provides almost all of the freshwater for the Keys, Miami, and the lower East Coast of Florida. Although a small part of the aquifer is beneath salty mangrove area, most of it is recharged by the freshwater Everglades. As sea level rises, saltwater will invade part of the Everglades, threatening both that ecosystem and the aquifer that lies beneath it.
Source: EPA (2002)

Water quality could suffer in areas experiencing increases in rainfall. For example, in the Northeast and Midwest increases in heavy precipitation events could cause problems for the water infrastructure, as sewer systems and water treatment plants are overwhelmed by the increased volumes of water.[1]Heavy downpours can increase the amount of runoff into rivers and lakes, washing sediment, nutrients, pollutants, trash, animal waste, and other materials into water supplies, making them unusable, unsafe, or in need of water treatment.[2]For information about how climate change and water quality affect public health, visit the Health Impacts & Adaptation page.

Freshwater resources along the coasts face risks from sea level rise. As the sea rises, saltwater moves into freshwater areas. This may force water managers to seek other sources of fresh water, or increase the need for desalination (or removal of salt from the water) for some coastal freshwater aquifers used as drinking water supply.[1]In addition, as more freshwater is removed from rivers for human use, saltwater will move farther upstream. Drought can cause coastal water resources to become more saline as freshwater supplies from rivers are reduced. Water infrastructure in coastal cities, including sewer systems and wastewater treatment facilities, faces risks from rising sea levels and the damaging impacts of storm surges.[2]

Impacts of Changes in Water Resources on Other Sectors

Warming air temperature can directly raise stream and lake temperatures, which can harm aquatic organisms that live in coldwater habitats, such as trout. Additionally, warmer water can increase the range of non-native fish species, permitting them to move into previously coldwater streams. The population of native fish species often decreases as non-native fish prey on and out-compete them for food.[2]For more information about how water quality impacts ecosystems, visit the Ecosystem Impacts & Adaptation page.

The impacts of climate change on water availability and water quality will affect many sectors, including energy production, infrastructure, human health, agriculture, and ecosystems.

Some regions of the United States, particularly the Northwest, use water to produce energy through hydropower. If climate change results in lower streamflows in areas where hydropower is generated, it will reduce the amount of energy that can be produced. Changes in the timing of streamflow can also have an impact on the ability to produce hydroelectricity. Lower water flows would also reduce the amount of water available to cool fossil-fuel and nuclear power plants. To learn more about climate change impacts on energy production, visit the Energy Impacts & Adaptation page.

Climate change impacts on water supply and quality will also affect tourism and recreation. The quality of lakes, streams, coastal beaches, and other water bodies that are used for swimming, fishing, and other recreational activities can be affected by changes in precipitation, increases in temperature, and sea level rise. In addition, winter sport activities that depend on the production of snow and ice could be limited in the future as temperatures increase.[1]For more information about how climate change may impact tourism and recreation, visit the Society Impacts & Adaptation page.

Agriculture and livestock depend on water. Heavy rainfall and flooding can damage crops and increase soil erosion and delay planting. Additionally, areas that experience more frequent droughts will have less water available for crops and livestock. To learn more about how climate change will impact agriculture and food production, visit the Agriculture and Food Supply Impacts page.[1]

Aquatic species that live in only coldwater environments, such as salmon, will be affected by rising water temperatures. Changing water temperatures would also affect the geographic range of fish species.[1]

Changes in the availability and quality of water are also major concerns for other countries where water resources are already stressed. For more information on these issues, please see the International Impacts & Adaptation page.

Planners across many sectors will confront the challenge of a changing water supply. They will likely adopt a variety of adaptation practices, designed to better conserve our water supplies, improve water recycling, and develop alternative strategies for water management. For more information, please visit the Water Resources Adaptation section.